Implicit body test with base64, file, hex are done on HTTP raw body response

[jcamiel]

Following #563, implicit body tests with base64, file, hex are executed against the raw maybe compressed body of the HTTP response, whereas they should be executed against the decompressed body.

# All tho tests are equivalent and test that the decompressed body is `Hello World!`
GET http://localhost:8000/compressed/gzip
HTTP/1.0 200
```Hello World!```

# Test base64 implicit body:
GET http://localhost:8000/compressed/gzip
HTTP/1.0 200
base64,SGVsbG8gV29ybGQh;  # `Hello World!` in base64

# Test hex implicit body:
GET http://localhost:8000/compressed/gzip
HTTP/1.0 200
hex,48656c6c6f20576f726c6421; # `Hello World!` in bytes

# Test file implicit body:
GET http://localhost:8000/compressed/gzip
HTTP/1.0 200
file,hello.txt; # `Hello World!` in a file